The present invention relates to a process for the laminar bonding of materials such as strips, foils or sheets, from a carrier material and at least one electroconductive and possibly compressed electroconductive powder coat, or from an electroconductive powder-based molded articles devoid of a carrier material wherein the carrier material and the powder coat or the powder-based molded article are exposed briefly to a magnetic alternating field in the frequency range from ca. 10 kHz to 120 MHz in order to generate in the powder coat or the powder-based molded article an induction current of such energy density that the points of contact of the powder particles among themselves and, if a carrier material is used, also their points of contact with this material are fused together at a temperature above the sintering temperature.
The powder must therefore at least in part consist of electroconductive components so that it is possible to generate an electrical current by induction. The structure of the powder, i.e. whether it is spherical, irregular, fibrous or whatever, is immaterial.
The present invention is e.g. oriented towards the laminar bonding of such materials as are used on a large scale as electrodes in electrolysers, batteries or fuel cells and as catalytically active elements in chemical processes. Power coats of the materials often contain one or more components susceptible to thermal impairment at high temperatures and subject to high demands with respect to the cohesion of the bonding, especially if during operation gases are generated within the possibly porous structure which exert considerable force on the structure and may cause cracks in the coating. The present invention particularly refers to e.g. the laminar bonding of porous powder coats with and without a carrier material for use as fillers, filter holders, catalysts, catalyst holders, diaphragms or membranes and sliding bearings, whereby the pores are filled with substances improving the behaviour of the bearings, e.g. their sliding behaviour. The present invention also applies to the making of soldered connections which gain extra strength by the additional fusion of the parts to be connected, and also applies to the production of coatings in which high-melting particles are embedded in a low-melting environment, as well to the fixation of electroconductive coatings manufactured according to production methods used for paper, foil and non-wovens.
In a process known from German Patent, DE 38 13 744 A1, the powder grains of powder coats are fixed to each other and fixed to a carrier material if used, fixation takes place by sintering in a reduced atmosphere, i.e. by a diffusion process at the points of contact of the powder grains at a temperature of the magnitude ⅔ to ⅘ of the absolute melting temperature in the case of metal one-component systems, and in all cases of high-melting components of powder mixtures considerably below the absolute melting point. This also applies to a process known from German Patent DE 30 15 981 A1 for the manufacture of highly porous self-baking electrodes for electrical accumulators, whereby metal particles are sintered together at a high temperature to form a porous structure on a strip carrier material and whereby instead of radiant heating, the powder-coated carrier strip is subjected to brief inductive heating in the magnetic field at sintering temperature. From European Patent EP 0 274 673 B1 a process is also known whereby a powder coat is fixed within itself and at its points of contact to a carrier material by sintering of the parts by means of induction in the magnetic alternating field.
The known sintering processes are time-consuming and cost-intensive and because of the long sintering time may impair the structure and properties of the powders used. As the powder coats are only bonded within themselves and with the carrier material by sintering of the points of contact, it results in a relatively weak cohesion of the powder coats and especially only limited adhesion of the powder coats to the carrier material. A further disadvantage with the known processes is that the entire mass of the powder coats and carrier material must be brought to and maintained at the sintering temperature for several minutes. As a result the risk of thermal impairment of temperature-sensitive powder components cannot be excluded.
A process is known from U.S. Pat. No. 5,389,408 for treatment of metallic particles which are deposited in a predetermined pattern on a non-metallic carrier in order to continuously produce a metallic conductor in the form of such a pattern. For this purpose, the particles are subjected to sufficient electromagnetic energy in order to fuse at least some of the particles. The temperature of the particles during the introduction of the electromagnetic energy is observed and the electromagnetic field shut off when the temperature reaches a predetermined value. In this method, a considerable time period of a number of seconds is required in order to fuse the fusible particles so that they build a flowable metallic smelt from which the desired continuous, homogenous metallic conductor strips are produced on solidification.
The object of this invention is to provide a process for the laminar bonding of materials of the kind referred to in the foregoing which is fast, cost-effective and preserves the structure of the powder. Good cohesion of self-supporting powder coats within themselves and, if a carrier material is used, good adhesion of the powder coat to the carrier material must moreover be assured.
The object is achieved, according to the present invention, such that in a process of the kind referred to above, the carrier material and the powder coat or the powder-based moulded article are exposed briefly to a magnetic alternating field in the frequency range of ca. 10 kHz to 120 MHz in order to generate in the powder coat or the powder-based moulded article an induction current of such energy density that the points of contact of the powder particles among themselves and, if a carrier material is used, also their points of contact with this material are used together at a temperature above the sintering temperature.
This process avoids a lengthy and structure-modifying sintering heat treatment. The process as per the present invention is independent of how the powder is applied to the carrier material and of the thickness of the powder coat or several powder coats so applied. The only condition is that the powder contain electroconductive particles so that an electrical current can be induced. Basically this is achieved by producing fusible particle surfaces at a temperature above the sintering temperature i.e. with induction currents of such energy density and of such short duration that a maim temperature gradient prevails at the core of the powder grain so that the core temperature is below the sintering temperature for a particle size above a minimum size in the micrometer range. Therefore, the temperature consciously chosen for the process is the melting temperature which is considerably higher than the sintering temperature and the process takes place at the points of contact of the powder particles. The nature and extremely short duration of the bonding process produces a strong bond with superiority in terms of mechanical values of a fused connection over a sintered connection without impairment of the porosity and shape retention of the bond by the higher bonding temperature. The high temperature difference between molten particle border zones and the particle core zones can be ensured by selecting a particle size above a minimum size in the micrometer range.
The technological advantages of the process as per the present invention are as follows: The fusing of the powder particles among themselves and with a carrier material results in a product with greater strength, better processability and above all with better adhesion of the powder coat to the carrier material than is possible with a sintering process. Temperature-sensitive powder components which can be thermally impaired at higher temperatures, better retain their essential properties because of the low core temperature of the powder particles, which can be ensured by an adequate particle size. The value of the minimum size to be selected depends on the grain structure and the properties of the material. For instance, the activity of Raney nickel powder which is often used as the catalytically active powder component in electrode coatings, clearly remains intact better with the process as per the present invention. In addition to these technological advantages there is the advantage of lower product manufacturing cost as against the known sintering process, because the product no longer has to undergo the time-consuming and costly diffusion process of-sintering in a protective atmosphere, but can instead be bonded inductively in a fraction of a second and in most cases even without a protective gas.
In the manufacture of bonded materials in strip form it is e.g. possible due to its short duration to incorporate the bonding process as per the present invention cost-effectively in a processing line with the other process steps, whereas for the sintering process in which a very long and correspondingly expensive sintering furnace must be used, this is only possible at considerable expense, because the sintering time is quite a bit longer than the other process steps.
Advantageous with the process as per the present invention for the bonding of materials in the form of self-supporting or substrate-fixed porous powder coats is a good cohesion of the powder coats in themselves and, if a supporting layer is used, good adhesion of the powder coat to the supporting layer. Thermal impairment of temperature-sensitive powder components and in some cases also a temperature-sensitive carrier can in many cases be excluded. Manufacturing costs are reduced as compared to the known processes.
With the process as per the present invention, in the laminar bonding of materials using powder coats with a carrier material the powder or powder mixtures can first be sprinkled in dosed quantity onto the carrier material, then evenly distributed and pressed down on it and if necessary compressed further. In laminar bonding without a carrier material, dosed quantities of the powder can e.g. be discharged into the nip between two horizontally juxtaposed rolls and upon passing through the nip be compressed into a flat layer. In both cases the powder coats at this stage have only a relatively low mechanical strength which is sufficient however to keep the powder coats together as they are being transferred to the induction binding plant.
The bonded material with or without carrier material can e.g. be exposed to the magnetic alternating field, if the magnetic field is focused linearly and a sufficiently high induction voltage is generated to obtain the required high energy density necessary for fusing in a very short time.
In order to achieve, in an advantageous embodiment of the present invention and with a product-dependent minimum particle size in the micrometer range, the induction current is generated with such a high energy density and for such a short time that only the points of contact of the powder particles among themselves and, if a carrier material is used, also their points of contact with the carrier material are fused together at a temperature above the sintering temperature without the core zones of the powder particles reaching a temperature at which the properties of the powder components could change.
Thereby a special effort is made to generate the induction current with such a high energy density and for such a short time that the core zones of the powder particles do not reach their sintering temperatures.
In a further advantageous embodiment or the process as per the present invention the induction current is generated with such a high energy density and for such a short time that although the points of contact of the powder particles among themselves and, if a carrier material is used, also their points of contact with that material are fused together, the porosity of the powder coat or the powdered-based moulded article remains intact.
When carrying out the process as per the present invention, the powder, in addition to the electroconductive component, may contain at least one further component which may be metallic or non-metallic.
The powder may e.g. contain non-electroconductive components provided that the electroconductive components form a cohesive structure in which the non-conductive components are embedded.
For the carrier material, preference is given to sheet metal strips or sheets, metal foils, hard paper, plastic foils, in each case perforated or unperforated, expanded metal, wire mesh, non-wovens or the like, in each case with or without an adhesive surface, the non-metallic carrier material in each case with and without a metallised surface.
It is also possible to reroll the induction-bonded materials in order to reduce their thickness and/or to smooth their surfaces and/or to profile them.
Further according to the present invention the bonding process takes place in the presence of selected gases e.g. an inert gas for oxidation suppression or an ionisable gas and therefore in a plasma.
It is further possible that the bonding process is supported by deoxidizing agents.
The thickness of the powder coat maybe a matter of millimeters, but may also be a matter of a few atom layers as is preferred for expensive noble metal catalysts on supporting layers. Such thin coats can e.g. be produced by electrodeposition or deposition by electrostatic means or from a suspension, or precipitated from a solution or may also be applied as a sludge or paste and then dried. In many cases the adhesion of the coat is supposed by adhesives.
Further objects, features, advantages and applications of this invention will be evident from the following description of embodiments with reference to the drawing. This means that all features described and/or pictorially represented, alone or in any combination, constitute the subject matter of the present invention and to be precise, this applies regardless of their summary in the individual claims or cross-references contained in them.